# 5 ways to better understand tractor stability

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Understanding a tractor’s stability — its center of gravity and stability baseline — in relation to outside forces could prevent a major accident or injury. Modern tractors and equipment have become increasingly safer than the older machines, but many farmers still continue to use older tractors on the farm.

These breakouts in tractor stability and instability from the Penn State University Extension will help you to better understand how to avoid tractor turnovers and related accidents.

1 Center of gravity
The center of gravity (CG) is the point on the tractor in which all parts balance one another. For a tractor to stay upright, the CG must stay within the stability baselines — imaginary lines drawn between points where the tractor’s tires touch the ground. While a tractor’s CG does not move, the stability baseline changes when the tractor moves from a level position onto a slope. When additional equipment is added, the CG shifts towards the weight of that equipment.

2 Centrifugal Force
Centrifugal force is the outward force nature exerts on objects or vehicles moving in a circular fashion — or the force trying to roll the tractor over while it is turning. This force increases as the turning angle becomes sharper and the speed of the tractor increases during a turn. Centrifugal force is often a factor in tractor side overturns. These overturns can occur when taking a turn too fast, traveling on a rough road where the tractor bounces and lands with its front wheels turned, or when over-correcting steering.

3 Rear-axle torque
Rear-axle torque involves energy transfer between the tractor engine and the rear axle of two-wheel drive tractors. Under normal circumstances, when the clutch is engaged, torque transferred to the tractor tires should move the tractor forward. If the rear axle is unable to rotate, the tractor chassis rotates around the axle causing the front-end to lift off the ground. This can happen when rear tires are frozen to the ground, stuck in the mud or blocked from rotating by the operator.

Because the CG is found closer to the rear axle, the tractor may only have to rear up 75 degrees before it passes its stability baseline and continues to roll over. This position is commonly called the “point of no return.”  A tractor may reach this position in 0.75 seconds, and it may take an operator longer than this to successfully stop the rearward motion. Four-wheel drive tractors are less susceptible to rear axle torque, however, once the front end lifts, there is little difference between two- and four-wheeled drive tractors.

4 Drawbar leverage
Drawbar leverage can also be a factor in rear overturns. When a two-wheel drive tractor is pulling a heavy load, the load puts weight on the rear tires and is pulling back on the tractor. This could cause the front end to lift off the ground. Tractors and drawbars are designed to counteract the tipping motion when the load is correctly hitched to the tractor. When the load is attached at any point other than its designed location, the design of the tractor for pulling loads is defeated. A tractor may also tip when the load is too much for the tractor.

Other examples that increase the likelihood of tipping a tractor while pulling a load include traveling up an incline too fast with a load that is too heavy. The load may dig into the ground forcing the tractor to rear up so fast, the result is a rear overturn.

5 ROPS
The rollover protective structure (ROPS) and seat belt, when worn, are the two most important safety devices to protect operators from death during tractor overturns, according to Penn State Extension. The ROPS does not prevent tractor overturns, but does protect the operator from being crushed during an overturn. A ROPS often limits the degree of rollover and a ROPS with enclosed cab can further limit a serious injury during a rollover. Wearing a seatbelt and staying within the ROPS frame is important for the ROPS system to be effective.

Source: Penn State University Extension, Tractor Stability and Instability

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